Despite being similar in appearance and behaviour to antelopes, the pronghorn actually belongs to its own unique family. The horns are particularly remarkable in that like bovids, they consist of a keratin sheath on a bony core, but like deer (cervids), they are forked, and the outer sheath sheds annually from the unforked bony core. Both sexes have black horns, but the male’s are enlarged and have forward-facing prongs below backward-pointing hooks, while the female’s are comparatively small. The pronghorn’s stocky body is supported on long, slim legs, which enable it to take massive eight metre strides at full speed. The upperparts are largely red-brown to tan, while the underparts, the rump and two bands across the neck are contrastingly white. The male also has conspicuous black patches on the face and on the sides of the neck, beneath the ears. Five subspecies, which differ slightly in appearance, are commonly recognised, with larger, darker forms living in the north, and smaller, lighter forms in the south.
Renowned for its speed, the pronghorn is the fastest terrestrial mammal in the Americas, capable of reaching top speeds of up to 86 kilometres per hour, and maintaining cruising speeds of 70 kilometres per hour for several kilometres at a time. It is active both day and night, with slight peaks just before sunrise and after sunset. Many plants feature in the pronghorn’s diet, including a variety of forbs, shrubs, grasses and cacti. Although it will drink freely if water is available, sufficient hydration can be derived from succulent plants if necessary. Daily movements vary depending on the seasons, with distances travelled during the winter typically being four times as far as during the summer.
During autumn and winter, the pronghorn often forms large, loose groups comprising as many as 1,000 individuals of all ages and both sexes. However, in spring and summer, these large herds segregate into much smaller groups segregated by sex. In wetter areas, where food is abundant, the mature males are territorial and, beginning in early March, will compete amongst each other for territories. To define a territory, males will scent mark with urine, faeces and secretion from glands behind the ears. Usually fights between territorial males will be resolved with a staring match and possibly some angry vocalisations and chasing, but when all else fails, rivals will resort to head to head fighting. During this time, the female groups, which may contain as many as 23 members, move freely between the territories of different males, whilst being pursued by bachelor herds of young males that move about on the edges of the areas controlled by the territorial males. However, when it comes to mating, which takes place during a period of about three weeks between July and early October, it is only the dominant males defending the most food-rich territories that sire offspring. In less productive, arid areas, the males dispense with forming territories, but instead attempt to defend groups of females in order to mate.
During the first pregnancy, females usually give birth to a single young after a gestation period of around 252 days, but subsequently, will often give birth to twins. Young at just two days old can already run faster than a horse, but do not have the stamina to keep up with the herd. As a result, for the first three to four weeks, the young remain hidden in vegetation, where they are safer from predators such as coyotes and golden eagles. Females continue to nurse and groom the young for four to five months, with both sexes reaching maturity at around 16 months old. Given that only dominant male’s breed, most will have to wait until they are three to five years of age before siring their first offspring.